Measuring the sample volatile content (which in many cases is the moisture content) is a frequent and repetitive chore in many analytical laboratories. In its simplest form, determining volatile or moisture content consists of weighing a representative sample of material, drying the material, then re-weighing the material to ascertain the losses on drying and, consequently, the initial volatile content of the sample. Convective, hot-air ovens, which are typically used for this task, can be relatively slow to bring the sample to "oven-dry" equilibrium. Such devices can also be expensive to operate as they inefficiently consume energy. These problems lessen the utility of hot-air devices for volatile analysis.
Drying substances using microwave energy to determine volatile or moisture content is generally convenient and precise. More importantly, microwave drying to measure moisture content is usually faster than equivalent hot-air methods. As in hot-air techniques, however, certain substances tend to burn, rather than merely become dry, when microwave power is applied to them. Stated differently, the rapid manner in which microwaves tend to interact with certain materials-which is an obvious advantage in some circumstances-can cause secondary heating of other materials that is disadvantageous (at least for volatile or moisture measurement purposes). Certain food products such as cheese are exemplary (although certainly not limiting) of materials that tend to burn rather than dry.
Additionally, microwaves interact with materials in a fashion known as "coupling," i.e., the response of the materials ("the load") to the microwave radiation. Some materials do not couple well with microwave energy, making drying or other volatile removal techniques difficult or imprecise. Other materials couple well when their moisture content or content of other microwave-responsive materials (e.g., alcohols and other polar solvents) is high. As they dry under the influence of microwaves, however, they couple less and less effectively; i.e., the load changes. As a result, the effect of the microwaves on the sample becomes less satisfactory and more difficult to control. In turn, the sample can tend to burn rather than dry, or degrade in some other undesired fashion. Both circumstances, of course, tend to produce unsatisfactory results.
As another factor, volatiles such as "loose" water (i.e., not bound to any compound or crystal) respond quickly to microwave radiation, but "bound" water (e.g., sodium carbonate monohydrate, Na.sub.2 CO.sub.3.H.sub.2 O) is typically unresponsive to microwave radiation. Instead, such bound water must be driven off thermally; i.e., by heat conducted from the surroundings. The nature of microwave radiation is such, however, that not all such surroundings may be heated when exposed to microwaves. Thus, the simple application of microwaves is typically less satisfactory for determining bound water than are more conventional heating methods.
Maintaining the sample's temperature below that which will cause pyrolysis helps prevent burning. To the extent moisture analysis is based upon weight measurements, however, measuring the sample's temperature cannot be permitted to hinder measuring the sample's weight. Thus, conventional direct probe temperature measurement is often unsatisfactory for this purpose. The temperature sensor's contact with the sample can lead to erroneous weight measurements when using sensitive weighing devices. Furthermore, in a microwave environment, the type, location, and function of a temperature probe must be compatible with the proper propagation of energy to, and absorption of energy by, the sample.
Some indirect methods have been developed to measure temperature in a way that avoids contacting the sample in an effort to avoid interfering with measuring sample weight. Many such indirect techniques, however, determine the temperature of something besides the sample itself. Thus, such techniques can lack accuracy, and can give a relatively slow response, particularly when heat conduction from the sample to the sensor is required.
Therefore, a need exists for taking advantage of the rapid response characteristics of microwave heating, while avoiding burning the sample, and while measuring the sample weight on a continuous basis that provides quick results.